1. Field of the Invention
The present invention relates to an apparatus for reproducing a digital signal recorded on a recording medium such as an optical disk, a magnetic disk, a magnetic tape or the like.
2. Background of the Invention
Viterbi Algorithm is known as one of methods for decoding a digital signal recorded at a high density on a recording medium with high reliability. In the Viterbi Algorithm, a read signal read from the recording medium is not simply identified as a binary value of 1 or 0 according to a predetermined threshold value, but sample values obtained by sampling read signals are detected as a continuous time-series, and a probable data series is obtained based on this-series.
A Viterbi decoder assumes state transition in a series of sample values successively supplied from an A/D convertor according to each estimated sample value stored in an estimated value memory (one of this status transitions is called branch, while continuous state transitions are called path), computes a branch-metric indicating probability of a branch as well as a path-metric indicating probability of a path, and decodes a probable data series according to the branch-metric and path-metric.
Namely, a Viterbi decoder computes a branch-metric and a path-metric according to same sample value stored in a sample value series supplied from an A/D convertor as well as to a plurality of estimated sample values stored in an estimated sample memory, and thus decodes a data series providing a minimum square error to an input data series to a regenerated digital signal. By executing Viterbi decoding, even if an S/N ratio of a read signal is low, data decoding can be executed at high reliability.
On the other hand, a optical pick-up in an optical disk reproducing apparatus can accurately track a recording track on an optical disk and read information therefrom, even if such a mechanical fluctuation such as a surface displacement, eccentricity, and inclination occurs, by executing focus servo and tracking servo. However, even if the servo system as described above is operating normally, sometimes a positional error which can not be followed due to a degree of mechanical fluctuation may remain. Also in some cases optical characteristics such as a reflection factor or a refractive index of the optical disk itself may change. If the residual error or change in the optical characteristics as described above occurs, an amplitude of a read signal fluctuates. For this reason, a sample value corresponding to the amplitude fluctuation is supplied to the Viterbi decoder in this step.
For this reason, in a binary value discrimination method in which discrimination for "0" or "1" is executed by comparing a given value to a specified threshold value, even if the amplitude fluctuation as described above is generated in a read signal, a digital signal can be regenerated without being affected by this phenomenon.
In Viterbi decoding, however, a level value itself of a read signal is used as a parameter for computing to decode a digital signal, so that, if amplitude fluctuation occurs in the read signal, the decoding performance is disadvantageously deteriorated.
A digital signal reproducing apparatus according to a first aspect of the present invention is a digital signal reproducing apparatus which obtains digital signals by reproducing recorded information from read signals read from a recording medium on which digital signals are recorded, and comprises an A/D convertor which successively sample the read signals and converts the signals to a digital sample value series, a sample value extracting means for extracting specified sample values from the sample value series, an offset effects by fluctuation of characteristics in the recording medium, a sample value extracting means for extracting a maximum sample value having a maximum level and a minimum sample value having a minimum level from sample values in the sample value series, an amplitude value detecting means for obtaining an amplitude value based on a result of subtraction of the minimum sample value from the maximum sample value and generating an amplitude signal according to the amplitude value, a dividing means for obtaining a result of division of a value corresponding to the amplitude signal by each sample value in the sample value series as an amplitude-corrected sample value, and a decoding means for obtaining a regenerated digital signal by means of executing decoding processing according to the amplitude-corrected sample value.
A digital signal reproducing apparatus according to a second aspect of the present invention is a digital signal reproducing apparatus which obtains digital signals by reproducing recorded information from read signals read from a recording medium on which digital signals are recorded, and comprises an A/D convertor which successively sample the read signals and converts the signals to a digital sample value series, a sample value extracting means for extracting a maximum sample value having a maximum level and a minimum sample value having a minimum level from sample values in the sample value series, an amplitude value detecting means for obtaining an amplitude value based on a result of subtraction of the minimum sample value from the maximum sample value and generating an amplitude signal according to the amplitude value, an estimated value memory for storing therein a plurality of estimated sample values each probable as a sample value in the sample value series, a multiplying means for obtaining a result of multiplication of uniformly multiplying each of the estimated sample value by a value corresponding to the amplitude signal as an amplitude-corrected estimated sample value, and a Viterbi decoder for decoding a data series providing an accumulative sum of square error values between samples values in the sample value series and the amplitude-corrected estimated sample value as the regenerated digital signal.
A digital signal reproducing apparatus according to a first feature of the present invention subjects a read signal read from a recording medium to A/D conversion to convert it to a digital sample value series, obtains an amplitude value according to a result of subtraction of a minimum value having a minimum level from a maximum level having a maximum level both in the sample value series, and obtains a corrected sample value with amplitude fluctuation having been corrected by uniformly dividing each sample value subjected to the A/D conversion by the amplitude value.
A digital signal reproducing apparatus according to a second feature of the present invention subjects a read signal read from a recording medium to A/D conversion to convert it to a digital sample value series, obtains an amplitude value according to a result of subtraction of a minimum sample value having a minimum level from a maximum sample value having a maximum level both in the sample value series, and supplies a result of cation by uniformly multiplying each of the estimated values in the Viterbi decoder by this amplitude value as a final estimated sample value to the Viterbi decoder.